FIG. 8 is a configuration example illustrating a conventional remote pumping system which is used in a wavelength division multiplexing optical fiber communication system (referring to K. Aida et al., Proc. Of ECOC, PDA-7, pp. 29-32, 1989 and N. Ohkawa et al., IEICE Trans. Commun., Vol. E81-B, pp. 586-596, 1998). In this remote pumping system, signal light is transmitted from a transmitting circuit 2 of a transmitter 1 and is received by a reception circuit 11 of a receiver 10 via three transmission fibers 5 to 7. Erbium-doped fibers (EDFs) 13F and 13R are positioned between the transmission fiber 5 and the transmission fiber 6 and between the transmission fiber 6 and the transmission fiber 7, respectively. Remote pumping light sources 3 and 13 are located in the transmitter 1 and receiver 10, respectively. The signal light is coupled to the pumping light supplied from the pumping light source 3, by a coupler 4. Furthermore, the signal light is coupled to the pumping light supplied from the pumping light source 13, by a coupler 14. Each of the transmitter 1, the receiver 10, and the pumping light sources 3 and 13 is connected to power source to be supplied with a power. The pumping light sources 3 and 13 adjacent to the transmitter 1 and the receiver 10 will be called a front stage pumping light source and a rear stage pumping light source, respectively. In addition, the pumping light supplied from the pumping light sources 3 and 13 will be called a forward pumping light and a backward pumping light, respectively. After passing through the transmission fiber 5, the forward pumping light pumps the EDF 13F. After passing through the transmission fiber 7, the backward pumping light pumps the EDF 13R.
Each pumping light has a wavelength near 1.48 μm which is appropriate for pumping the EDF. The signal light outputted from the transmitter 1 is attenuated in the transmission fiber 5 and is amplified in the EDF 13F. Furthermore, the signal light is attenuated in the transmission fiber 6 and is amplified in the EDF 13R. After passing through the transmission fiber 7, the signal light is received by the receiver 10. Under such circumstances, it is possible to un-repeatedly transmit the signal light over the entire length of the transmission fibers 5, 6, and 7, without supplying power on the way. In comparison to a repeating system which does not use the remotely pumped EDFs 13F and 13R, there is an advantage in greatly enlarging the un-repeated distance, namely, repeater spacing, in the above-mentioned remote pumping. Incidentally, it is possible to adopt either one of a configuration using the forward pumping light source 3 and EDF 13F and a configuration using the backward pumping light source 13 and EDF 13R. In addition, a certain degree of distributed gain (Raman gain) is given to the signal light, since the signal light is generally subjected to Raman amplification in the transmission fiber in which the pumping light is propagated.
In the conventional remote pumping system illustrated in FIG. 8, the pumping light, which reaches the EDFs 13F and 13R, travels from the pumping light input ends of EDFs 13F and 13R to the pumping light output ends of EDF 13F and 13R that are positioned at opposite sides of the pumping light input ends, since the gain wavelength region is set to C band (1530 nm to 1560 nm) of EDFA in the signal light. As a result, the pumping light pumps the EDFs 13F and 13R across the entire fiber lengths, respectively.
However, it has been found that the pumping light, which reaches the EDFs 13F and 13R, only pumps to the vicinities of pumping light input ends of EDFs 13F and 13R and almost none travels to the opposite pumping light output ends, in the case where the gain wavelength region is set to L band (1570 nm to 1600 nm) of EDFA in the signal light. Since the EDF for the L band has a length which is several times as long as the length of EDF for C band, the pumping light reaches only the vicinity of the pumping light input end. In addition, absorption occurs in the part of EDF that is not pumped, except in the vicinity of the pumping light input end. As a result, there is a problem in that the pumping efficiency is reduced in each of the EDFs 13F and 13R and the noise figure increases in each of the EDFs 13F and 13R. Incidentally, the L band is a gain wavelength region for signal light and is as important as the C band. More particularly, the L band is an important gain wavelength region for signal light since it is possible to prevent the four-wave mixing, which is a problem in the C band, in a system using a dispersion-shifted fiber (DSF).